The present invention relates to a semiconductor memory including a ferroelectric capacitor and a method for driving the same.
A semiconductor memory including a ferroelectric capacitor is expected to work as a nonvolatile memory capable of providing a limitless read number.
A conventional semiconductor memory including a ferroelectric capacitor will now be described with reference to FIG. 8.
As shown in FIG. 8, a source region 2 and a drain region 3 are formed in surface portions of a silicon substrate 1. On a region of the silicon substrate 1 sandwiched between the source region 2 and the drain region 3, a silicon oxide film 4, a ferroelectric film 5 of a metal oxide such as lead zirconate titanate (PZT) or bismuth tantalate strontium (SBT) and a gate electrode 6 are successively formed, so as to together form a ferroelectric FET.
In this structure, the polarization direction of the ferroelectric film 5 can be set to the upward direction or the downward direction, and the depth of surface potential of a region of the silicon substrate 1 below the gate electrode 6 can be set to two different states respectively corresponding to the two polarization states (namely, the upward polarization and the downward polarization). Since the depth of the surface potential corresponds to the resistance between the source and the drain of the ferroelectric FET, the resistance between the source and the drain is set to either a high value or a low value depending upon the polarization direction of the ferroelectric film 5. Since the upward polarization or the downward polarization is kept (stored) as far as the polarization of the ferroelectric film 5 is kept, the ferroelectric FET can be used as a nonvolatile memory.
In the ferroelectric FET having this structure, a state where the ferroelectric film 5 has the downward polarization is allowed to correspond to, for example, a data xe2x80x9c1xe2x80x9d and a state where it has the upward polarization is allowed to correspond to a data xe2x80x9c0xe2x80x9d. For example, when a ground potential is applied to the lower face of the silicon substrate 1 with a large positive voltage applied to the gate electrode 6, the polarization of the ferroelectric film 5 can be set to the downward polarization. Alternatively, when a ground potential is applied to the lower face of the silicon substrate 1 with a large negative voltage applied to the gate electrode 6, the polarization of the ferroelectric film 5 can be set to the upward polarization. After setting the polarization of the ferroelectric film 5 to the downward or upward polarization, the potential of the gate electrode 6 is restored to the ground potential.
FIGS. 9A, 9B and 9C are energy band diagrams obtained when the silicon substrate 1 has p-type conductivity and the source region 2 and the drain region 3 have n-type conductivity. FIG. 9A shows the energy band obtained when the polarization is downward (namely, a data xe2x80x9c1xe2x80x9d is stored), FIG. 9B shows the energy band obtained when the polarization is upward (namely, a data xe2x80x9c0xe2x80x9d is stored) and FIG. 9C shows the thermal equilibrium energy state. In FIGS. 9A through 9C, a reference numeral 11 denotes the conduction band of the gate electrode 6, a reference numeral 12 denotes the energy band of the ferroelectric film 5, a reference numeral 13 denotes the energy band of the silicon oxide film 4, a reference numeral 14 denotes the energy band of the silicon substrate 1 and a reference numeral 15 denotes the energy band of a depletion layer formed in the vicinity of the surface of the silicon substrate 1. Also, a void arrow denotes the polarization direction of the ferroelectric film 5.
In the case of the downward polarization (corresponding to a data xe2x80x9c1xe2x80x9d), the negatively ionized depletion layer 15 extends to a deep region of the silicon substrate 1 as shown in FIG. 9A, and hence, the surface potential of the silicon substrate 1 becomes lower than the ground potential.
In the case of the upward polarization (corresponding to a data xe2x80x9c0xe2x80x9d), no depletion layer is formed in the silicon substrate 1 because holes, that is, p-type carriers, are stored on the surface of silicon substrate 1 as shown in FIG. 9B, and hence, the surface potential of the silicon substrate 1 accords with the ground potential.
Since the surface potential of the region of the silicon substrate 1 below the gate electrode 6 thus depends upon the polarization direction, when a potential difference is caused between the drain and the source, a current flowing between the drain and the source is different depending upon the polarization direction. Specifically, when the surface potential of the silicon substrate 1 is lower than the ground potential (namely, when a data xe2x80x9c1xe2x80x9d is stored), the resistance between the drain and the source is low (namely, the FET is in an ON state), and hence, a large current flows between the drain and the source. On the other hand, when the surface potential of the silicon substrate 1 accords with the ground potential (namely, when a data xe2x80x9c0xe2x80x9d is stored), the resistance between the drain and the source is high (namely, the FET is in an OFF state), and hence, substantially no current flows between the drain and the source. When a current value between the drain and the source is detected, it can be found whether the ferroelectric FET is in the state corresponding to a data xe2x80x9c1xe2x80x9d or in the state corresponding to a data xe2x80x9c0xe2x80x9d.
Since whether the ferroelectric FET is in the state corresponding to a data xe2x80x9c1xe2x80x9d or in the state corresponding to a data xe2x80x9c0xe2x80x9d can be thus found, the polarization of the ferroelectric film 5 is not reversed in reading a data from the ferroelectric FET. Thus, what is called a non-destructive read-out system is realized. In other words, there is no need to carry out an operation for recovering the direction or the magnitude of the polarization, namely, a rewrite operation, after data read.
In this manner, a ferroelectric FET is capable of a non-destructive read operation, and therefore, a problem of polarization fatigue of a ferroelectric film, which is caused in a destructive read operation accompanying polarization reversal, can be avoided. Accordingly, the ferroelectric FET is expected to work as a nonvolatile memory capable of providing a limitless read number.
However, the ferroelectric film 5 of the ferroelectric FET is generally a semiconductor having a large number of defective levels, and hence, electrons and holes can easily move within the ferroelectric film 5.
Therefore, when the ferroelectric FET is in an ON state as shown in FIG. 9A, since electrons are injected from the conduction band 11 of the gate electrode 6 into the ferroelectric film 5, charge at the head of the polarization is neutralized and hence the bottom of the potential in a V shape is gradually elevated, resulting in the thermal equilibrium energy state shown in FIG. 9C.
On the other hand, when the ferroelectric FET is in an OFF state as shown in FIG. 9B, since holes are injected from the conduction band 11 of the gate electrode 6 into the ferroelectric film 5, charge at the head of the polarization is neutralized and hence the apex of the potential in a reverse V shape is gradually lowered, also resulting in the thermal equilibrium energy state shown in FIG. 9C.
As a result, since the surface potential of the silicon substrate 1 becomes the same level in spite of the different polarization directions, namely, the upward polarization and the downward polarization, it is difficult to distinguish the two states depending upon a current flowing between the drain and the source.
This problem can be explained by using a hysteresis curve 20 of the ferroelectric capacitor and a gate capacitance load line 21 of the ferroelectric FET drawn on a polarizationxe2x80x94voltage (Q-V) plane shown in FIG. 10. The structure of the ferroelectric FET of FIG. 8 can be regarded as a series circuit of the ferroelectric capacitor and a metalxe2x80x94oxide filmxe2x80x94silicon (MOS) capacitor when a virtual electrode is disposed between the ferroelectric film 5 and the silicon oxide film 4.
In this series circuit, in the case of the downward polarization (namely, the state of storing a data xe2x80x9c1xe2x80x9d corresponding to the energy band diagram of FIG. 8A), the polarization is positioned in a point 22 immediately after data write, and a negative bias voltage corresponding to a distance from the origin O to the point 22 is applied to the ferroelectric film 5. This bias voltage causes electron injection into the ferroelectric film 5, and therefore, the polarization moves from the point 22 to the origin O.
On the other hand, in the case of the upward polarization (namely, the state of storing a data xe2x80x9c0xe2x80x9d corresponding to the energy band diagram of FIG. 8B), the polarization is positioned in a point 23 immediately after data write, and a positive bias voltage corresponding to a distance from the origin O to the point 23 is applied to the ferroelectric film 5. This bias voltage causes hole injection into the ferroelectric film 5, and therefore, the polarization moves from the point 23 to the origin O.
Thus, in the conventional ferroelectric FET, a difference between a data xe2x80x9c1xe2x80x9d and a data xe2x80x9c0xe2x80x9d is distinguished by using a potential difference induced between the ferroelectric film 5 and the silicon oxide film 4 depending upon the polarization direction. However, the potential difference induced between the ferroelectric film 5 and the silicon oxide film 4 works as a driving force for injecting electrons or holes for eliminating the induced potential difference. In other words, there is a problem that the voltage is unavoidably eliminated through the electron or hole injection into the ferroelectric film 5.
In consideration of the aforementioned conventional problems, an object of the invention is reading a data stored in a ferroelectric capacitor even when a potential difference caused in the ferroelectric capacitor is eliminated through electron or hole injection.
In order to achieve the object, the first semiconductor memory of this invention comprises a ferroelectric capacitor formed on a semiconductor substrate and including a ferroelectric film, a first electrode formed on the ferroelectric film and a second electrode formed under the ferroelectric film; data writing means for causing a first state in which the ferroelectric film has polarization in a direction from the first electrode to the second electrode or in a direction from the second electrode to the first electrode and has a substantially saturated polarization value or causing a second state in which the ferroelectric film has polarization in the same direction as in the first state and has a substantially zero polarization value, whereby writing a data corresponding to the first state or the second state in the ferroelectric capacitor; and data reading means for reading a data stored in the ferroelectric capacitor by detecting whether the ferroelectric capacitor is in the first state or in the second state.
In the first semiconductor memory, two different states of the ferroelectric capacitor storing different data (such as a data xe2x80x9c1xe2x80x9d and a data xe2x80x9c0xe2x80x9d) are distinguished from each other by the two states in which the polarization of the ferroelectric film is in the same direction, namely, the first state in which the ferroelectric film has the substantially saturated polarization value (corresponding to, for example, a data xe2x80x9c1xe2x80x9d) and the second state in which the ferroelectric film has the substantially zero polarization value (corresponding to, for example, a data xe2x80x9c0xe2x80x9d). Accordingly, even when a potential difference caused in the ferroelectric capacitor is eliminated through electron or hole injection, a data stored in the ferroelectric capacitor can be read.
In the first semiconductor memory, the data writing means preferably causes the first state or the second state in the ferroelectric capacitor by applying a voltage between a first signal line connected to the first electrode and a second signal line connected to the second electrode.
Thus, an operation for writing a data xe2x80x9c1xe2x80x9d or a data xe2x80x9c0xe2x80x9d in the ferroelectric capacitor by causing the first state or the second state in the ferroelectric capacitor can be easily and directly carried out.
In the first semiconductor memory, the data reading means preferably includes means for generating, between the first electrode and the second electrode, a voltage for inducing, in the ferroelectric film, an electric field in the same direction as the direction of the polarization of the ferroelectric film.
Thus, the direction of the polarization of the ferroelectric film is not changed even when a reading voltage is applied to the ferroelectric capacitor, and therefore, the ferroelectric capacitor can keep on storing a data. As a result, the polarization fatigue of the ferroelectric film can be reduced.
In the first semiconductor memory, the data reading means preferably includes a load capacitor serially connected to the ferroelectric capacitor and means for applying a reading voltage to both ends of a series circuit composed of the ferroelectric capacitor and the load capacitor, and detects whether the ferroelectric capacitor is in the first state or in the second state by detecting a voltage induced in the load capacitor resulting from dividing the reading voltage in accordance with a ratio between capacitance of the ferroelectric capacitor and capacitance of the load capacitor.
Thus, a read operation can be eased because a data stored in the ferroelectric capacitor can be read by detecting the voltage induced in the load capacitor in applying the reading voltage to the both ends of the series circuit of the ferroelectric capacitor and the load capacitor.
In the first semiconductor memory, the data reading means preferably includes a field effect transistor formed on the semiconductor substrate and connected to the second electrode at a gate electrode thereof and means for applying a reading voltage between the first electrode and the semiconductor substrate or a source electrode of the field effect transistor, and detects whether the ferroelectric capacitor is in the first state or in the second state by detecting a change of channel conductance of the field effect transistor caused by a voltage induced in the gate electrode resulting from dividing the reading voltage in accordance with a ratio between capacitance of the ferroelectric capacitor and gate capacitance of the field effect transistor.
Thus, a read operation can be eased because change of the voltage induced in the gate electrode of the field effect transistor in applying the reading voltage between the first electrode and the semiconductor substrate or the source electrode of the field effect transistor can be detected as change of the channel conductance of the field effect transistor.
In the first semiconductor memory, the data reading means can include a bit line connected to the second electrode and means for applying a reading voltage between the first electrode and the bit line, and detect whether the ferroelectric capacitor is in the first state or in the second state by detecting a voltage induced in the bit line resulting from dividing the reading voltage in accordance with a ratio between capacitance of the ferroelectric capacitor and capacitance of the bit line.
The second semiconductor memory of this invention comprises a memory cell block of a plurality of successively connected memory cells each including a ferroelectric capacitor having a ferroelectric film, a first electrode formed on the ferroelectric film and a second electrode formed under the ferroelectric film and a cell selecting transistor serially connected to the ferroelectric capacitor; data writing means for causing a first state in which the ferroelectric film of a selected ferroelectric capacitor selected by the cell selecting transistor from the plurality of ferroelectric capacitors has polarization in a direction from the first electrode to the second electrode or in a direction from the second electrode to the first electrode and has a substantially saturated polarization value or causing a second state in which the ferroelectric film of the selected ferroelectric capacitor has polarization in the same direction as in the first state and has a substantially zero polarization value by applying a writing voltage between a control line connected to a first common node out of two common nodes included in the memory cell block and a second common node out of the two common nodes, whereby writing a data corresponding to the first state or the second state in the selected ferroelectric capacitor; and data reading means, including a load capacitor connected to the first common node and means for applying a reading voltage between the second common node and the load capacitor, for detecting whether a selected ferroelectric capacitor selected by the cell selecting transistor from the plurality of ferroelectric capacitors is in the first state or in the second state by detecting a voltage induced in the load capacitor resulting from dividing the reading voltage in accordance with a ratio between capacitance of the selected ferroelectric capacitor and capacitance of the load capacitor, whereby reading a data stored in the selected ferroelectric capacitor.
In the second semiconductor memory, two different states of the ferroelectric capacitor storing different data (such as a data xe2x80x9c1xe2x80x9d and a data xe2x80x9c0xe2x80x9d) are distinguished from each other by the two states in which the polarization of the ferroelectric film is in the same direction, namely, the first state in which the ferroelectric film has the substantially saturated polarization value (corresponding to, for example, a data xe2x80x9c1xe2x80x9d) and the second state in which the ferroelectric film has the substantially zero polarization value (corresponding to, for example, a data xe2x80x9c0xe2x80x9d). Accordingly, it is possible to realize a memory cell array in which a data stored in the ferroelectric capacitor can be read even when a potential difference caused in the ferroelectric capacitor is eliminated through electron or hole injection.
The first method for driving a semiconductor memory of this invention comprises the steps of writing a data in a ferroelectric capacitor formed on a semiconductor substrate and including a ferroelectric film, a first electrode formed on the ferroelectric film and a second electrode formed under the ferroelectric film; and reading a data stored in the ferroelectric capacitor, and the step of writing a data includes a sub-step of causing a first state in which the ferroelectric film has polarization in a direction from the first electrode to the second electrode or in a direction from the second electrode to the first electrode and has a substantially saturated polarization value or causing a second state in which the ferroelectric film has polarization in the same direction as in the first state and has a substantially zero polarization value, whereby writing a data corresponding to the first state or the second state in the ferroelectric capacitor, and the step of reading a data includes a sub-step of detecting whether the ferroelectric capacitor is in the first state or in the second state, whereby reading a data stored in the ferroelectric capacitor.
In the first method for driving a semiconductor memory, two different states of the ferroelectric capacitor storing different data (such as a data xe2x80x9c1xe2x80x9d and a data xe2x80x9c0xe2x80x9d) are distinguished from each other by the two states in which the polarization of the ferroelectric film is in the same direction, namely, the first state in which the ferroelectric film has the substantially saturated polarization value (corresponding to, for example, a data xe2x80x9c1xe2x80x9d) and the second state in which the ferroelectric film has the substantially zero polarization value (corresponding to, for example, a data xe2x80x9c0xe2x80x9d). Accordingly, even when a potential difference caused in the ferroelectric capacitor is eliminated through electron or hole injection, a data stored in the ferroelectric capacitor can be read, and polarization fatigue of the ferroelectric film can be reduced.
In the first method for driving a semiconductor memory, the step of writing a data preferably includes a sub-step of causing the first state or the second state in the ferroelectric capacitor by applying a voltage between a first signal line connected to the first electrode and a second signal line connected to the second electrode.
Thus, an operation for writing a data xe2x80x9c1xe2x80x9d or a data xe2x80x9c0xe2x80x9d in the ferroelectric capacitor by causing the first state or the second state in the ferroelectric capacitor can be easily and directly carried out.
In the first method for driving a semiconductor memory, the step of reading a data preferably includes a sub-step of reading a data stored in the ferroelectric capacitor by detecting whether the ferroelectric capacitor is in the first state or in the second state after setting a potential of the second signal line to a ground potential and placing the second electrode in a floating state by disconnecting the second electrode from the second signal line.
When the potential of the second signal line is thus once set to the ground potential, the potential of the second electrode can be defined. Therefore, unnecessary charge stored in the second electrode during a write operation or a read operation conducted before this read operation can be removed. Furthermore, when the reading voltage is applied after placing the second electrode in the floating state by disconnecting the second electrode from the second signal line, it can be definitely detected whether the ferroelectric capacitor is in the first state or in the second state.
In the first method for driving a semiconductor memory, the step of reading a data preferably includes a sub-step of generating, between the first electrode and the second electrode, a voltage for inducing, in the ferroelectric film, an electric field in the same direction as the direction of the polarization of the ferroelectric film.
Thus, the direction of the polarization of the ferroelectric film is not changed even when the reading voltage is applied to the ferroelectric capacitor, and hence, the ferroelectric capacitor can keep on storing a data.
In the first method for driving a semiconductor memory, the step of reading a data preferably includes a sub-step of applying a reading voltage to both ends of a series circuit composed of the ferroelectric capacitor and a load capacitor serially connected to the ferroelectric capacitor, and detecting a voltage induced in the load capacitor resulting from dividing the reading voltage in accordance with a ratio between capacitance of the ferroelectric capacitor and capacitance of the load capacitor, whereby detecting whether the ferroelectric capacitor is in the first state or in the second state.
Thus, a read operation can be eased because a data stored in the ferroelectric capacitor can be read by detecting the voltage induced in the load capacitor in applying the reading voltage to the both ends of the series circuit of the ferroelectric capacitor and the load capacitor.
In the first method for driving a semiconductor memory, the step of reading a data preferably includes a sub-step of applying a reading voltage between the first electrode and the semiconductor substrate or a source electrode of a field effect transistor formed on the semiconductor substrate and connected to the second electrode at a gate electrode thereof, and detecting change of channel conductance of the field effect transistor caused by a voltage induced in the gate electrode resulting from dividing the reading voltage in accordance with a ratio between capacitance of the ferroelectric capacitor and gate capacitance of the field effect transistor, whereby detecting whether the ferroelectric capacitor is in the first state or in the second state.
Thus, a read operation can be easily and definitely carried out because the voltage induced in the gate electrode of the field effect transistor in applying the reading voltage between the first electrode and the semiconductor substrate or the source electrode of the field effect transistor can be detected by detecting change of the channel conductance of the field effect transistor.
In the first method for driving a semiconductor memory, the step of reading a data can include a sub-step of applying a reading voltage between the first electrode and a bit line connected to the second electrode, and detecting a voltage induced in the bit line resulting from dividing the reading voltage in accordance with a ratio between capacitance of the ferroelectric capacitor and capacitance of the bit line, whereby detecting whether the ferroelectric capacitor is in the first state or in the second state.
The first method for driving a semiconductor memory preferably further comprises, at least in the case where the ferroelectric capacitor is in the second state, a step of setting a potential of the second electrode to a ground potential after removing the reading voltage applied to the first electrode in the step of reading a data.
Thus, the polarization of the ferroelectric film is restored to the state before data read, and therefore, even when the ferroelectric capacitor is in the second state, the data read operation can be repeatedly carried out.
The second method for driving a semiconductor memory of this invention comprises the steps of writing, in a memory cell block of a plurality of successively connected memory cells each including a ferroelectric capacitor having a ferroelectric film, a first electrode formed on the ferroelectric film and a second electrode formed under the ferroelectric film and a cell selecting transistor serially connected to the ferroelectric capacitor, a data in a selected ferroelectric capacitor selected by the cell selecting transistor from the plurality of ferroelectric capacitors; and reading a data stored in a selected ferroelectric capacitor selected by the cell selecting transistor from the plurality of ferroelectric capacitors, and the step of writing a data includes a sub-step of applying a writing voltage between a control line connected to a first common node out of two common nodes included in the memory cell block and a second common node out of the two common nodes, and causing a first state in which the ferroelectric film of the selected ferroelectric capacitor has polarization in a direction from the first electrode to the second electrode or in a direction from the second electrode to the first electrode and has a substantially saturated polarization value or causing a second state in which the ferroelectric film of the selected ferroelectric capacitor has polarization in the same direction as in the first state and has a substantially zero polarization value, whereby writing a data corresponding to the first state or the second state in the selected ferroelectric capacitor, and the step of reading a data includes a sub-step of applying a reading voltage between the second common node and a load capacitor connected to the first common node, and detecting whether the selected ferroelectric capacitor is in the first state or in the second state by detecting a voltage induced in the load capacitor resulting from dividing the reading voltage in accordance with a ratio between capacitance of the selected ferroelectric capacitor and capacitance of the load capacitor, whereby reading a data stored in the selected ferroelectric capacitor.
In the second method for driving a semiconductor memory, two different states of the ferroelectric capacitor storing different data (such as a data xe2x80x9c1xe2x80x9d and a data xe2x80x9c0xe2x80x9d) are distinguished from each other by the two states in which the polarization of the ferroelectric film is in the same direction, namely, the first state in which the ferroelectric film has the substantially saturated polarization value (corresponding to, for example, a data xe2x80x9c1xe2x80x9d) and the second state in which the ferroelectric film has the substantially zero polarization value (corresponding to, for example, a data xe2x80x9c0xe2x80x9d). Accordingly, even when a potential difference caused in the selected ferroelectric capacitor is eliminated through electron or hole injection, a data stored in the ferroelectric capacitor selected in a memory cell array can be read.